Various arrangements of electrical relays are known in the prior art for switching electrical signals. In one example of the prior art, a multiplexor arrangement of interconnected reed type relays provides for flexibly configuring the relays along electrical propagation paths coupled with selected I/O ports, so as to advantageously route the electrical signals from a large number of input/output (I/O) ports of the multiplexor, while limiting a number of the relays required.
For example, data acquisition applications may require monitoring a respective resistance value of each member of a large number of sensors. The multiplexor arrangement of interconnected relays provides for flexibly configuring the relays along electrical propagation paths coupled with selected I/O ports, so as to advantageously route the electrical signals between a large number of sensor monitoring input ports of the multiplexor and one or more output ports coupled with a meter for measuring the respective resistance value of each sensor.
While prior art switching matrices provide some advantages, they also include some limitations. For some reed type relays, over an operating temperature range between -40 degrees Centigrade and +85 degrees Centigrade, there is an individual thermal offset voltage drop of ten to thirty microvolts through each reed type relay that varies up to %500 in relation to the operating temperature of the reed type relay. At any given operating temperature, there still may be wide variability in thermal offsets of various reed type relays. Furthermore, for each configuration of the relays along a respective electrical propagation path, a respective accumulated thermal offset corresponds to summation of the individual thermal offsets of the relays of each configuration. Accordingly, if there are differing numbers of configured relays along various propagation paths, then there may be differing accumulated thermal offsets for the various propagation paths. This is undesirable since identical resistances of sensors would be misreported and measured as different, depending on the differing accumulated thermal offsets for the propagation paths used in the measurements.
Such difficulties become worse as numbers of the I/O ports increase and a corresponding complexity of the switching multiplexor increases. For example, so called four wire measurements provide some advantages in additional measurement sensitivity, primarily by attenuating contact resistance errors, but four wire measurements require twice as many I/O ports as two wire measurements, and increase switching multiplexor complexity. Four wire measurements use two wires to drive current through a resistance to be measured, and two additional wires to sense a corresponding voltage drop though the resistance. Since four wire measurements are more sensitive than two wire measurements, they are also more susceptible to the errors introduced by differing accumulated thermal offsets.
While four wire measurements are important, configuration flexibility for providing single wire measurements is also important.
What is needed is method and apparatus using a switching multiplexor arrangement of relays having configuration flexibility and operative for balancing thermal offset of the relays.